Let's look at the hand step by step, starting pre-flop.
They are playing heads-up effectively 24BB deep. This deep heads-up it could be a good strategy to limp with your entire range on the button, which will be 90-95% of hands. A limp with J⋄5♠ on the button from caecilius seems good and a check from BrynKenney makes a lot of sense as well. Raising from the big blind doesn't make a lot of sense. At this stackdepth you are never really bluffing from the big blind pre-flop and Q♥7♣ is simply to weak to do so. Note: going to the flop both players have super wide ranges.
Flop comes 5♥8♥6♣, BrynKenney checks the big blind and I like this play. Although there isn't really a pre-flop aggressor, BrynKenney will be checking it over to caecilius on most of the flops. Betting with Q7 doesn't really make a lot of sense. Q high has showdown value, you have the backdoor flush draw and an open-ended straight draw, so a lot of equity, but just not enough to get it all-in at this SPR. Getting raised after you bet the flop just really sucks and by checking BrynKenney avoids this situation. Q7 is for sure good enough to check call.
Caecilius checks it back, which is pretty standard with bottom pair. Even though some protection would be nice, especially without a heart in your hand, on a flop this wet. You are incredibly vulnerable for check-raises. Therefore you should be checking most of your bottom pairs here.
A♥. This is an interesting turn card. BrynKenney could decide to start bluffing here. But again, he really doesn't want to get check-raised, because he has so much equity. Caecilius has plenty of aces in his range that checked the flop and on a wet board like this, caecilius could definitely raise it up with most of them. Better bluff candidates would be hands BrynKenney doesn't mind folding to a large raise. Like any heart, preferably a low heart or 7x without a heart and/or without showdown value, and then every sort of single straight draw. Plenty of other bluffs BrynKenney can have in this spot. I like BrynKenney's play.
Caecilius check it back again, which is again pretty standard. He just has showdown value and is looking to check it back on the river.
5♣. BrynKenney missed his draws. He can't really win the hand at this point. Should he check-fold, bet or check-raise? I personally don't like check-folding. BrynKenney should have a bluffing range on every street. His hand here is pretty much perfect. Betting on the flop and the turn with this exact hand did not seem so good, but on the river he can definitely do something. How many other hands can he really bluff with here? Not too many. Now should he check-raise or bet?
Let's take a look at caecilius's range first here. Most of the times I think his range is a pair of fives, a pair of sixes or king high. BrynKenney's has good blockers. He blocks the occasionally slow-played flush or straight. Betting definitely seems reasonable. You can represent a five or a flush that play this way and put king high and a pair of sixes in a tough spot. I would probably have chosen to do this.
The next level play here is to check-raise. What are some of the advantages and disadvantages of this play? Let's start with the disadvantages. Caecilius for sure checks back king high and a pair of sixes on the river, which BrynKenney could have bluffed him off of. There are a lot of advantages to this play though. For once, having some check-raises here with both bluffs and value hands makes you tough to play against. Secondly, your opponent's betting range on the river is, let's be honest, pretty weak. Almost all of his flush draws on the flop would have bet and most of his straights and sets would have bet the flop as well, if not that then the turn, because the board was so draw heavy. Most aces would be betting the turn for that same reason. Realistically caecilius can only really value bet a weird 8 or a 5. You can probably check-raise him off those hands, while they would call a river bet most of the time. Note: because caecilius overbets the river he is primarily representing trips.
BrynKenney brilliantly breaks the hand down and shoves the river. He knows his opponent is weak. And he can credibly represent a flush, which could have checked the flop and slowplayed the turn and river. BrynKenney blocks caecilius from having a weird flush or straight, so this is pretty much the best hand to do it with after Kh7x.
I think both betting and check-raising the river are good with this hand. But mixing in both is definitely superior and only the best of the best are capable of actually implementing this.
Now back to caecilius, should he call the river? At this point trips is a bluff catcher. BrynKenney is definitely good enough to be balanced here. So I think caecilius should just take a look at his range and fold the bottom part of it and call the top part. He overbet the river, so his value range should be like 53s, 54s, 75s, 95s, T5, J5, Q5, K5 and some weird not frequent slowplayed flushes, straights and full houses. Caecilius probably wouldn't even overbet with his weaker fives, since BrynKenney just calls him with a better five too often. So J5 is right in the middle, if not near the bottom of his range. Given the fact he also does not have a heart blocker in his hand, I think this is a pretty clear fold.
I haven't talked about caecilius' river sizing. Now I think caecilius' range is more capped than BrynKenney's. BrynKenney can have flushes and full houses here, that slowplayed the turn knowing caecilius will bet the turn frequently and also slowplayed the river. Caecilius can realistically not really have those hands. I don't really understand the overbet, since he is just putting himself into a stupid spot. He can get raised by flushes and full houses and can even get called by a better five. All worse hands will likely fold to this overbet. By betting small you can still get value from a pair of eights or sixes.
Overbetting only really makes sense when your opponent's range is capped, not yours. However, caecilius is a great player and I would love to hear his reasoning.